The present invention relates to a motor driving apparatus for driving a motor by impressing a DC voltage to the motor through a PWM (Pulse-Width Modulation) control according to a rotation speed setting information.
As an example of motor driving apparatus of this type, a driving apparatus for a commutator less DC motor is known. This apparatus has motor drive windings of a fixed three-phase Y-connection, a permanent magnet rotor, and three-phase bridge-connected six switching elements as a commutator. A DC power source is impressed to the motor drive windings via the switching elements. Each switching element is made on/off by commutation control signals given based on a position detection of the permanent magnet rotor, for example, based on induced voltages which are induced in the motor drive windings. By this, a motor current is commutated, and the permanent magnet rotor rotates. The commutation control signals given to each switching element are PWM-controlled according to a rotation speed setting information, that is, a pulse width of commutation control signals are varied by PWM control according to the rotation speed setting information. By this, an average voltage impressed to the motor drive windings, namely a rotation speed of the motor is varied.
By the way, in such a driving apparatus, a maximum rotation speed of the motor is limited by a voltage of the DC power source which is impressed to the switching elements composing the commutator. That is, since the induced voltages in the motor drive windings are increased accordingly as the rotation speed of the motor is increased, when the induced voltages reach a voltage which is given by the commutation control signals with 100% duty ratio, no current flows into the motor drive windings and the rotation speed stops increasing further. Hence, from the viewpoint of expanding a rotation sphere of the motor, a high voltage is desired as a voltage of the DC power source.
However, when the voltage of the DC power source is set high based on the above-mentioned viewpoint, the rotation area of the motor is expanded, but as a high DC voltage is impressed to the motor via PWM chopping, a variation in motor current becomes large particularly in low and medium-speed rotation zones. Because of this, there is a problem that a hysteresis loop due to variation in magnetic flux becomes large, causing an increase in iron loss.
FIG. 8 is an explanatory drawing for explaining an increase in iron loss. In FIG. 8, a waveform (a) shows a current flowing through the motor drive windings and a waveform (b) shows a voltage impressed to the motor drive windings. When the rotation speed of the motor is low, a pulse width of the commutation control signals given to the switching elements via PWM chopping is decreased, and a width of the voltage pulse impressed to the drive windings is also decreased. Therefore, as shown in FIG. 8, the motor current (a) is a sawtooth wave with a large amplitude, causing an increase in current variation. Because of this, if the voltage of the DC power source is set high, a voltage value of the voltage pulse impressed to the drive windings is high, the motor current (a) is also large and the current variation is increased further. Therefore, if the voltage of the DC power source is set high, the hysteresis loop due to variation in magnetic flux becomes large, and the iron loss is increased further.